1. Field of the Invention
The present invention relates to a manufacturing method and a manufacturing apparatus for a flat image display device including a pair of substrates which are opposed to each other and are attached to each other at their peripheral edge parts.
2. Description of the Related Art
In recent years, various image display devices have been developed as next-generation light-weight, small-thickness display devices, which will take the place of cathode-ray tubes (hereinafter, referred to as CRTs). Such image display devices include liquid crystal displays (LCDS) which control the intensity of light by making use of alignment of liquid crystal, plasma display panels (PDPs) which cause phosphors to emit light by ultraviolet of plasma discharge, field emission displays (FEDs) which cause phosphors to emit light by electron beams of field-emission-type electron emitting elements, and surface-conduction electron-emitter displays (SEDs) which cause phosphors to emit light by electron beams of surface-conduction-type electron emitting elements.
The FED or SED, for example, generally comprises a front substrate and a rear substrate that are opposed to each other across a predetermined gap. These substrates have their respective peripheral portions joined together by a sidewall in the form of a rectangular frame, thereby forming a vacuum envelope. A phosphor screen is formed on the inner surface of the front substrate. Provided on the inner surface of the rear substrate are a large number of electron emitting elements for use as electron emission sources, which excite the phosphors to luminescence.
A plurality of support members are provided between the rear substrate and the front substrate in order to support an atmospheric-pressure load acting on these substrates. The rear substrate-side potential is substantially set at a ground potential, and an anode voltage is applied to the phosphor surface. Electron beams, which are emitted from the electron emitting elements, are applied to red, green and blue phosphors of the phosphor screen, and cause the phosphors to emit light. Thereby, an image is displayed.
According to the FED or SED constructed in this manner, the thickness of the display device can be reduced to about several millimeters, so that the device can be made lighter in weight and thinner than CRTs that are used as displays of existing TVs or computers.
For the FED, for example, various manufacturing methods have been examined to join the front substrate and the rear substrate that constitute the envelope by means of the sidewall in the form of a rectangular frame. In general, a sintering material such as frit glass is filled between the two substrates and the side wall, and the sintering material is heated and sintered in a furnace. Thus, the substrates and the side wall are coupled to form the envelope. In an example of the basic procedure, a structure, in which the rear substrate and side wall are coupled by fusion, is prepared in advance, and the front substrate is joined to this structure.
However, when frit glass is sintered, unnecessary gas is produced. The gas remains in the sealed envelope after fusion, and the gas causes a problem when the inside of the envelope is evacuated later to a high vacuum level. Jpn. Pat. Appln. KOKAI Publication No. 2002-319346, for instance, discloses another method. In this method, a low-melting-point sealing material, such as indium, is filled between the front substrate and rear substrate. Then, current is supplied to the sealing material in a vacuum apparatus, and the sealing material itself is heated and melted by the resulting Joule heat to seal substrates together (hereinafter referred to as “electric heating”). According to this method, only the sealing material can be heated up to high temperatures and melted. Thus, a long time is not needed to heat and cool the substrates, and the substrates can be joined to form the envelope in a short time.
In the case of the electric heating, however, it is necessary to supply current so as to stably melt the sealing material. If the sealing material is not stably melted, the time for melting the sealing material varies from envelope to envelope, and stable coupling of the substrates cannot be carried out. If the electrically conductive sealing material is excessively heated, such problems arise that the sealing material may be broken due to heat or a crack may occur in the substrates. Conversely, if the sealing material is not sufficiently melted, the coupling of the substrates becomes deficient, and such problems arise that the air-tightness for maintaining vacuum deteriorates or the vacuum state of the envelope cannot be kept. Under the circumstances, in the prior art, a DC current of 100 A is supplied to the entire sealing material, and heating/melting is carried out for about one minute. Thereby, the sealing material is stably melted. On the other hand, 10 to 20 minutes are needed for cooling. In order to improve mass-productivity, there has been a demand for a further decrease in sealing time.
Although the time for melting and cooling the electrically conductive sealing material can be reduced by increasing the value of the constant current, the increase in current value leads to frequent occurrence of sparks between the sealing material and the electrode, between the electrode and the apparatus-side electrode contact, or between the sealing layers, and there arises the problem that the sealing layer cannot stably be melted.
In addition, in the above-described manufacturing method, only one side of the substrate, to which the indium is applied, is heated by the power-supply heating, resulting in a difference in temperature between the front and back surfaces of the substrate. Consequently, such a warp occurs on the substrate that the surface, on which the indium is applied, becomes convex. In this case, after cooling, the corner portions of the envelope become thicker than the central parts of the side portions of the envelope. If the envelope becomes partly thick, such problems arise that the air-tightness for vacuum deteriorates, the relative position between the electron source and phosphor layer is displaced at the corner part, and the envelope cannot easily be attached to the cabinet.